Helical spring fastener

ABSTRACT

The invention is a two-ended fastener having one end suitable for attaching the fastener to a base with a second end having a special thread that is substantially cylindrical at its root and sharp at its crown. This thread is designed to mate with the radially inward surfaces of a round wire helical spring so that the helical spring may simply be threaded to the fastener. By utilizing the special thread described, a high degree of surface contact between the fastener and the spring is made possible, increasing spring retention. The special thread inner diameter may be made slightly larger than the interior diameter of the spring so that the helical spring will be slightly expanded when threaded to the fastener. An elastic force between the threaded portion of the spring and the fastener further helps retain the spring to the fastener. This elastic force increases as an attempt is made to unthread the spring from the fastener.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains broadly to the field of fasteners and inparticular to fasteners designed to retain a helical spring.

2. Description of the Related Art

Prior methods of retaining a helical spring have included placing oneend of the spring over a fixed cylinder and then retaining the spring onthe cylinder by way of clamps, two-piece collars, screws,brazing/welding or one or more pins placed through coils of the springand the accompanying cylinder. Another technique includes using atapered cylinder or mandrel upon which an end of the spring is pressedwith the compression of the spring on the mandrel retaining the springin place.

Of course, the need to fasten helical springs is varied. In one militaryapplication, a need arises where it is desired to affix a spring-basedantenna onto a vehicle or other surface. Many military vehicles such asJeeps, HUMVEEs, tanks, river patrol boats and the like oftentimesincorporate such antennas for communication purposes. The need extendsnot only to surface vehicles but to submersibles such as mannedsubmarines and underwater robotic vessels. In the latter application,such a fastener should be able to hold a helical spring both whenvibrated and when subjected to air and water flow. It is also desirablethat a spring fastener be easy to manufacture, easy to implement and lowin cost.

SUMMARY OF THE INVENTION

The invention is a two-ended fastener having one end suitable forattaching to a base such as a vehicle or antenna with a second endhaving a special exterior thread that is substantially cylindrical atits root and sharp at its crown. This thread is designed to mate withthe radially inward surfaces of a round wire helical spring so that thehelical spring may simply be threaded onto the fastener. By utilizingthe special thread described, a high degree of surface contact betweenthe fastener and the spring exists. This thread design prevents thespring from easily disengaging from the fastener.

In a variation of the invention, the inner thread diameter of thespecial thread may be made slightly larger than the inner diameter ofthe spring so that the helical spring will be slightly expanded whenthreaded to the fastener. An elastic force will then exist between thethreaded portion of the spring and the fastener enhancing the retentionof the spring on the fastener. This elastic force increases when anattempt is made to unthread the spring from its threaded positionfurther increasing the spring retention to the fastener.

OBJECTS OF THE INVENTION

It is an object of this invention to provide an improved means ofretaining a helical spring.

Another object of the invention is to provide a fastener for retaining around-wire helical spring that is easy to manufacture, easy toimplement, that is simple in design and low in cost.

Yet another objective of the invention is to provide a method forretaining a spring-based antenna.

Other objects, advantages and new features of the invention will becomeapparent from the following detailed description of the invention whenconsidered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C are side, top and detailed views, respectively, of thefastener of the invention (dimensions shown in inches).

FIGS. 2A and 2B are respectively side and top views of a round-wirehelical spring as may be utilized in the invention (dimensions ininches).

FIG. 3 is an exploded view of a utilization of the invention in aspring-based antenna scheme.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1A, a side view of a representative example of theinvention is shown This example is like one utilized with an underwaterrobot incorporating a spring-based antenna. For this application, thefastener of the invention was fabricated from Acetal PolyoxymethylineAcetal plastic. This plastic provided ample strength for the conditionsexpected and was impervious to saltwater corrosion. In FIG. 1A it can beseen that fastener 10 has a first end 12 that permits the fastener to beattached to a base such as a vehicle body or such as to a collarattached to an antenna whip. In the embodiment shown, end 12 wasthreaded with a 1.500-18UNC-2A thread. Fastener 10 also includes asecond end 14 in which a special thread 16 has been made. In the actualimplementation of the invention this thread was cut into the Acetalplastic utilizing a tool that mimics the radial inward surface of amatching round-wire helical spring.

In FIGS. 2A and 2B such a round-wire helical spring, 18, is shown.Spring 18 has radially inward surfaces 20 that match special thread 16.In the specific implementation of the invention, spring 18 was made fromspring steel of Cres-type 302 having a spring-wire diameter of 0.188inches per AMS 5688 and a spring temper in accordance with QQ-W-423 inwhich the finish was passivated in accordance with QQ-P-35, type VI orVII. The spring had 26 coils, was righthanded, and had its ends ground.Initial tension was 7 pounds plus or minus 1 pound. Such a spring isavailable through Moore Spring Company of Spring Valley, Californiaunder part number 71591-188.

FIG. 1B is a top view of fastener 10 with FIG. 1C showing a detail ofthe special threading utilized in the invention. In FIG. 1C it canreadily be seen that threading 16 is conical at its root and sharp atits crown. This threading permitted maximum interfacing with radiallyinward surfaces 20 of spring 18 so as to enhance the spring retentioncapability of fastener 10.

In the specific implementation of the invention used, the insidediameter of thread 16 was made approximately 0.003 of an inch largerthan the inside diameter of spring 18. This was done so that as spring18 was threaded onto fastener 10 a slight expansion of the springoccurred at the spring's threading interfaces. This, of course, causedan elastic force between fastener 10 and the threaded portion of spring18, further enhancing the spring retention capability of the fastener.

This elastic force is of particular importance where rotational forceswork upon spring 18, posing a possible disassembly of spring 18 fromfastener 10. It has been found that a rotational force in the directionof unthreading spring 18 from fastener 10 actually increases thespring's grip upon the fastener. During the application of this force,the spring radially compresses to cause this effect. Applying arotational force in the opposite direction simply further tightens thespring to the fastener.

Referring now to FIG. 3 there is shown an exploded view of aspring-based antenna assembly 22. In such an assembly two fastener,shown as 10' and 10", are utilized with round-wire helical spring 18.First fastener 10' has a first end 24 that is suitably attached to abody 26 by threading, welding or other sufficient attachment mechanism(threading being shown). Round-wire helical spring 18 is then screwedonto second end 28 of first fastener 10'. The spring is screwed ontocylindrical thread 29 of the fastener until seated against its flange30. As previously explained, the special threading of the fastener maybe designed so that an expansion of spring 18 occurs at the spring'sinterface with the fastener's threads. The resulting compression of thespring upon fastener 10 enhances spring retainment.

In a similar fashion, second fastener 10" is used to attach an antennawhip to spring 18. In this scenario whip 32 is fitted into a collar 34having a recess 36 suitably dimensioned to accept end 38 of the whip.Whip 32 is then fixed in place by any suitable means such as screw-infasteners 40. Collar 34 is also designed to attach to second fastener10". In the implementation shown, first end 42 of fastener 10" hasthreads that match with threads 44 suitably tapped within collar 34Second fastener 10" has, like first fastener 10', a second end, 46,defining conical threads 48 suitable for matching the radially inwardsurfaces 20 of spring 18.

It can readily be seen that the fastener of the invention may beutilized in a wide variety of scenarios where it is desired that ahelical spring be retained. The one-part nature of the fastener isconsidered superior to its multiple-part competitors. It is easy tomanufacture, easy to use, simple in design and low in cost. Thissingle-part design can be considered more reliable than manymultiple-part retention systems as there are no screws, clamps, etc., towork loose. In addition, the fastener of the invention fits inside autilized spring, resulting in a very small package, which is, at most,of only a slightly larger diameter than the outside diameter of thespring used.

A number of different utilizations of the invention are, of course,possible. The material used to produce the invention could be changed toother suitable materials such as stainless steel, aluminum, bronze aswell as many plastics. Of course, the diameter, size and nature of thespecial threads used in the invention could be changed to accommodatemany different springs.

Obviously, many modifications and variations of the invention arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as has been specifically described.

What is claimed is:
 1. A fastener for retaining a round wire helicalspring comprising:a first end for attaching said fastener to a base; anda substantially cylindrical second end for attaching said fastener tosaid helical spring, said second end having an exterior thread that iscylindrical at its root and sharp at its crown and that substantiallyconforms to radially inward surfaces of said helical spring, saidexterior thread further having an inner diameter that is slightly largerthan an inner diameter of said round wire helical spring so that saidhelical spring is slightly expanded where threaded to said fastener. 2.An flexible link comprising:a round wire helical spring; and a fastenerattached at both ends of said round wire helical spring, each of saidfasteners having an exterior thread substantially cylindrical at itsroot and sharp at its crown that mates with radially inward surfaces ofsaid round wire helical spring, with said exterior thread further havingan inner diameter that is slightly larger than an inner diameter of saidhelical spring so that said helical spring is slightly expanded wherethreaded to said fasteners.
 3. An antenna system comprising:a round wirehelical spring having a first and second end; a fastener having a firstend for attaching said fastener to a base and having a second end forattaching said fastener to said first end of said round wire helicalantenna spring, said second end of said fastener having an exteriorthread substantially cylindrical at its root and sharp at its crown thatmates with radially inward surfaces of said round wire helical spring;an antenna whip; a collar having a recess into which an end of saidantenna whip is fixed; a second fastener having a first end forattaching to said collar and having a second end for attaching to saidsecond end of said round wire helical antenna spring, said second end ofsaid second fastener having an exterior thread substantially cylindricalat its root and sharp at its crown that mates with radially inwardsurfaces of said round wire helical spring, and in which said exteriorthread of said fasteners have an inner diameter that is slightly largerthan an inner diameter of said helical spring so that said helicalspring is slightly expanded where threaded to said fasteners.